Simple polysaccharides, such as bacterial levan (BL), not conjugated to protein elicit a thymus independent (TI) response. Although the TI response is clonally restricted, the repertoire of antibodies that can be produced is much greater as evidenced by the appearance of new clonotypes produced in mice immunized with polysaccharide-protein complexes. The focus of this investigation is on the mechanisms or events that determine which of the potential antibodies is actually expressed. Our earlier studies examining the isoelectric focusing (IEF) pattern of IgG antibodies produced in response to immunization with BL in genetically defined mouse strains demonstrated that the complexity of the IgG response is regulated by at least one C57BL/6 gene, designated spectrotype regulation gene 1 (Sr1 ), and is not linked to the Igh gene complex, MHC, or coat color. Work in progress is focused on mapping the Sr1 gene to a murine chromosome. A large group of (BALB/c X C57BL/6)F1 mice backcrossed to BALB/c has been bred, immunized with BL, and phenotyped as Sr1 positive or negative by IEF analysis of anti-inulin antibodies in the serum. Spleen DNA has been isolated from all of the backcross 1 (BC1) mice. Using simple sequence repeat (SSR)-PCR, the BC1 mice have been genotyped at genetic markers located on chromosomes 1 through 19. Our data indicate linkage between Sr1 and an autosome and this result is supported by phenotype and genotype analyses of CXB recombinant inbred lines. Genotyping of the BC1 mice and additional CXB strains is in progress. We plan to breed additional BC1 mice and examine the genotype and phenotype of a congenic strain which lacks C57BL/6 genes in the region we have identified as a possible location for Sr1. Recent studies with TCRb/TCRd double knockout mice have confirmed that the response to BL is a TI response. Mapping and characterization of a gene (Sr1 ) which regulates the expression of antibody diversity following immunization with a polysaccharide antigen will contribute to a betterunderstanding of the immune response to polysaccharides and of the regulation of antibody diversity.